“Currently insured individual” defined

The term “currently insured individual” means any individual who had not less than six quarters of coverage during the thirteen-quarter period ending with (1) the quarter in which he died, (2) the quarter in which he became entitled to old-age insurance benefits, (3) the quarter in which he became entitled to primary insurance benefits under this subchapter as in effect prior to August 28, 1950 , or (4) in the case of any individual entitled to disability insurance benefits, the quarter in which he most recently became entitled to disability insurance benefits, not counting as part of such thirteen-quarter period any quarter any part of which was included in a period of disability unless such quarter was a quarter of coverage, and who satisfies the criterion specified in subsection (c).